feat(amneziawg): Phase 2a — IPv6 support + NDP proxy

Adds native dual-stack IPv6 to AmneziaWG inbounds, ported from
coinman-dev/3ax-ui's approach:

- ServerSettings gets ipv6Enabled/ipv6Subnet/ipv6ExternalInterface;
  Instance carries the server's own IPv6 address (first host of the
  subnet) alongside its IPv4 one.
- defaultAmneziaWGClients allocates an IPv6 host address per client
  (second AllowedIPs entry) when the server has IPv6 enabled, reusing
  allocateWireguardAddress — which needed a real fix along the way: it
  always suffixed "/32" regardless of address family, which is wrong
  for an IPv6 host address (needs /128). Now family-aware.
- generateServerConfig's PostUp/PostDown gains IPv6 forward-accept
  rules, proxy_ndp sysctl, and one `ip -6 neigh add/del proxy` entry per
  enabled peer with an IPv6 address — the lightweight per-client
  method, not the ndppd-daemon whole-subnet method (not worth the
  config-file-management complexity at this scale; ndppd itself is
  still installed by install.sh in case that changes later).
- ValidateIPv6Subnet rejects a malformed subnet before save.
- Frontend: ipv6Enabled/ipv6Subnet/ipv6ExternalInterface fields on the
  AmneziaWG inbound form, EN+RU translations, openapi.json/generated/*
  regenerated (the latter via `go run ./tools/openapigen`, pure Go).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/web/translation/ru-RU.json

@@ -1816,6 +1816,11 @@
         "secondaryDns": "Резервный DNS",
         "externalInterface": "Внешний интерфейс",
         "externalInterfaceHint": "Сетевой интерфейс хоста для NAT (PostUp/PostDown). Оставьте пустым для автоопределения.",
+        "ipv6Enabled": "Включить IPv6",
+        "ipv6Subnet": "Подсеть IPv6",
+        "ipv6SubnetHint": "Например, fd86:ea04:1115::/64. Обязательно при включённом IPv6.",
+        "ipv6ExternalInterface": "Внешний интерфейс для IPv6",
+        "ipv6ExternalInterfaceHint": "Сетевой интерфейс хоста для записей NDP-прокси. Оставьте пустым, чтобы использовать «Внешний интерфейс».",
         "obfuscation": "Параметры обфускации",
         "regenerateObfuscation": "Сгенерировать заново",
         "jc": "Jc (кол-во мусорных пакетов)",